Code Division Multiple Access (CDMA) is a well-known scheme for multiplexing communication channels that is based on the method of direct sequence spread spectrum [1]. CDMA is often incorporated into electronic communication networks, especially in cellular communication, and is considered to be superior to other traditional multiplexing schemes, such as Time Division Multiple Access (TDMA) wherein the entire bandwidth is available to a channel for a short slice of time and Frequency Division Multiple Access (FDMA) wherein only a part of the bandwidth is available to a channel all the time.
In CDMA, the entire bandwidth is available to all channels all the time. Each channel has a unique key that identifies it, thereby enabling an information receiver to discriminate between the channels. Such a key is a pseudo-noise sequence with a bandwidth that is much larger than that of the input data. In electronic communications, the key sequence is known in advance at both the transmitter and the receiver sides.
FIGS. 1A-1C schematically illustrate the CDMA operation principle. FIG. 1A shows a CDMA transmitter scheme: a transmitter multiplies an input data signal by a key sequence, thus spreading the spectrum of the input data signal and causing it to appear as noise in itself. FIG. 1B shows a CDMA receiver scheme: to extract the data “out of the noise”, the received output is multiplied by the conjugate key sequence (the key itself if the key sequence is real). FIG. 1C shows the CDMA spectral characteristic. It is clear that if the receiver key is not the correct one, or if it is not well synchronized with the transmitter, the multiplication by the conjugate key sequence does not reveal the data and only yields a broadband noise-like result.
Thus, many channels can be multiplexed over the same bandwidth by using a different key for each channel. The effect of all other channels on a given channel is reflected only in the noise level at the receiver. Hence, keys with good auto-correlation and cross-correlation properties are to be used in order to minimize the noise level. Ideally, the key should imitate the correlation properties of band-limited white noise and should be as long as possible.
CDMA has several advantages over conventional methods. First, CDMA is well adapted to dynamic changes of the number of simultaneously operating channels. Specifically, when one channel becomes inactive, the other channels benefit from the fact that the noise level is reduced. Thus, an allocated channel in CDMA that is not transmitting at a given time, automatically “frees its space” to other channels that need the bandwidth at that time. Second, CDMA is inherently flexible to dynamic changes in the bit rate and the quality of service (signal to noise ratio) of any channel without affecting the total amount of data transmitted by all the channels. This is due to the fact that in CDMA, the resource allocated per channel is power (as opposed to time or bandwidth in other methods). Hence, if a channel is allowed to transmit more power, it can either improve the quality of service or increase the bit rate of that channel. Consequently, this shared resource (power) can be dynamically allocated between the channels, and any channel can dynamically trade bit rate for quality of service and vice versa at a given power. Third, in CDMA, all channels are equivalent, so the quality of service is that of the average channel, while in other methods, the quality of service is dictated by the worse channel.
In optical communication, the available optical bandwidth is much larger than what can be supported by current electronic modulators. Thus, in order to utilize efficiently the available bandwidth, optical multiplexing of several electronic channels is required. The CDMA approach is thus most attractive for this purpose, and attempts have been made to incorporate optical CDMA into optical communication networks [14-20]. Various solutions for the optical CDMA schemes have also been disclosed in the following patent publications: U.S. Pat. No. 4,866,699, U.S. Pat. No. 5,177,768, U.S. Pat. No. 5,867,290, U.S. Pat. No. 6,236,483, U.S. 2002/0163696; WO 00/29887; U.S. 5,784,506; U.S. Pat. No. 6,025,944.
The major problem for obtaining optical CDMA is that of generating the pseudo-noise key. Since the key should be much broader in bandwidth than the data, and since the data bandwidth in optical communication is already close to the limit that electronic modulators can support, it is impossible to generate the key electronically and it is necessary to generate the key optically. The many attempts to solve this problem can be divided to two categories—a coherent approach and an incoherent approach. The coherent approach [14-16] starts from a broadband coherent source, i.e. a mode locked laser that emits transform-limited pulses, where the phase of all frequencies is known to be zero. The key for each channel is then generated by actively shaping the phases of the different frequencies in a unique manner through some kind of a pulse-shaping device, which deforms the pulse to mimic a pseudo-noise burst. At the receiver system, a shaper performs the inverse shaping to recreate the original transform limited pulse, which is then detected. This approach suffers from sensitivity to dispersion and to non-linear effects in the fiber, and more important, from the fact that a lot of the flexibility of CDMA is lost due to the limitations imposed by active pulse shaping (e.g., the total number of channels is limited by the number of pixels of the pulse shaper and the lowest effective bit rate per channel is limited by the spectral resolution of the shaper).
The incoherent approach (with its many versions) [15, 17-20] involves an incoherent broadband source. Although such a source emits “true noise”, the phase of the emitted field is not known, so only intensity manipulations are possible. This makes the incoherent approach robust in the sense that it is relatively immune to phase changes due to propagation effects. However, since the incoherent approach is inherently unipolar, the cross correlation of different keys cannot average out to zero. Thus, the existence of many channels contributes not only noise, but also background DC intensity, which causes the signal to noise ratio and the performance to deteriorate severely [15-20]. For this reason, the capacity of incoherent CDMA systems is inherently and significantly lower than that of coherent systems (√{square root over (N)} channels compared to N channels in the coherent approach).
The known CDMA techniques utilize complicated algorithms to pre-design practical keys that approximate the characteristics of the desired ideal white-noise key. These approximations are usually constrained by other design considerations, for example, the tradeoff between key length and design simplicity, yielding a non-optimal result.